Imagers typically consist of an array of pixel cells containing photosensors. Each pixel cell produces a signal corresponding to the intensity of light impinging on its photosensor when an image is focused on the array by one or more lenses. These signals may be stored in a memory and displayed on a monitor, manipulated by software, printed, or otherwise used to provide information about the optical image. The magnitude of the signal produced by each pixel is substantially proportional to the amount of light impinging on its photosensor.
Several kinds of imagers are generally known. Complementary metal-oxide-semiconductor (“CMOS”) imagers and charge coupled device (“CCD”) imagers are among the most common. CMOS imagers are discussed, for example, in U.S. Pat. No. 6,140,630, U.S. Pat. No. 6,204,524, U.S. Pat. No. 6,310,366, U.S. Pat. No. 6,326,652, and U.S. Pat. No. 6,376,868, which are all commonly assigned to Micron Technology, Inc.
To allow an imager to capture a color image, the pixel cells must be able to separately detect color values of light, for example, red (R) light, green (G) light, and blue (B) light. A color filter array is typically placed in front of the array of pixel cells so each pixel cell measures light of the color of its respective filter. The most common type of color filter array, often referred to as a “Bayer filter,” is described in U.S. Pat. No. 3,971,065. Bayer filters consist of alternating red and green filters in a first row and alternating blue and green filters in a second row. This pattern is repeated throughout the array. Thus, in an imager employing a Bayer filter, one-half of the pixels are sensitive to green light, one-quarter are sensitive to red light, and the remaining one-quarter are sensitive to blue light.
To provide a color image, however, red, green, and blue values are required for every pixel location. Thus, the two “missing” color values at each pixel location must be estimated. For example, a pixel underlying a red portion of a Bayer filter directly measures red light but does not measure blue and green light. Therefore, blue and green values must be estimated for this red pixel location. Similarly, red and green values must be estimated for blue pixel locations, and red and blue values must be estimated for green pixel locations. This estimation process is often referred to as “demosaicing.”
New imager designs often contain more pixel cells than previous generation imagers in order to provide increased image resolution. To maintain small imager size, however, the size of each pixel cell is often reduced from one generation to the next. This reduces the photosensitive area associated with each pixel cell, making the image sensor more susceptible to noise, especially under low light conditions. Noise can be reduced by applying mathematical transforms, often called “filters,” to pixel values.
Prior art noise reduction techniques separately filter red, green, and blue pixel values. This can result in blurring or inefficient noise reduction. Therefore, an improved method of reducing noise in Bayer-domain images is desirable.